Solar cells are one of the most promising energy sources available. However, further improvement needs to be made in their efficiency to make them more economically competitive with other energy sources. The invention presented herein provides solar cell technology that dramatically, surprisingly and unexpectedly improves solar cell efficiency.
Solar cells have two general architectural backbone configurations known as “substrate” and “superstrate”. A superstrate configuration usually comprises a transparent substrate, a transparent conductive oxide layer (TCO) for a “top” electrode, a window layer, an absorber layer, a back contact layer, and a “bottom” or “back” electrode. Conventionally CdTe solar cells are manufactured in a superstrate configuration with transparent substrates such as rigid glass. A “substrate” configuration has a different architecture, a substrate, a back electrode layer, a back contact layer, an absorber layer, a window layer and a transparent conductive oxide (TCO) layer. A substrate configuration is required when opaque substrates such as flexible metal foils are used for high volume production of CdTe devices.
The solar cell electrical contact at the interface between the absorber layer and the opaque-side electrode is known as the back contact is known as the “back contact”. The back contact must meet certain criteria that are generally understood in the art for device optimum performance. For example it is preferred that the interface is prepared such that the current voltage (I-V) curve of the device is substantially linear and symmetric, which is the signature of an ohmic contact. To form an ohmic contact with p-type semiconductors, the top of the semiconductor valence band has to align with the Fermi level of the metal. This necessitates an electrode metal with a work function higher than the electron affinity plus the band gap of the absorber material. For CdTe, a preferred absorber material the work function needs to be greater than about 6 eV.
The difficulty in providing ohmic contacts to semiconductor absorber thin films is that there is no practical metal with a work function greater than about 6 eV thus making it extremely difficult to produce an ohmic contact.
One way the prior art addresses the problem is by forming pseudo-ohmic contacts which requires treating a Group II-VIA semiconductor surface with various chemical etchants. Chemical etching methods are difficult to control and if uncontrolled have the potential to etch grain boundaries of the absorber semiconductor which can reduce the efficiency of the solar cells. Also, and most important in a substrate configuration solar cell, a thin layer cannot be created by the prior art treatment of Group II-VIA semiconductor compound layer surfaces, since the Group II-VIA semiconductor compound layer is deposited after the metal deposition and not accessible for such treatment. Another prior art solution to forming ohmic contacts between a CdTe absorber layer and the back electrode material in superstrate CdTe solar cells is using a film of ZnTe on CdTe, see Studies of ZnTe Back Contacts to CdS/CdTe Solar Cells T. A. Gessert, P. Sheldon, X. Li, D. Dunlavy, D. Niles, R. Sasala, S. Albright and B. Zadler. Presented at the 26th IEEE Photovoltaic Specialists Conference, September 29B. October 1997, the contents of which are incorporated herein by reference in its entirety. Another prior art solution in superstrate devices is using copper-doped ZnTe on CdTe, see “Development of Cu-doped ZnTe as a back-contact interface layer for thin-film CdS/CdTe solar cells.” T. A. Gessert, A. R. Mason, P. Sheldon, A. B. Swartzlander, D. Niles, and T. J. Coutts. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films May 1996 Volume 14, Issue 3, pp. 806-812, the contents of which are hereby incorporated by reference in its entirety. V. P. Singh et al./Solar Energy Materials & Solar Cells 59 (1999) 145-161 suggested using a thin layer (about 50 nm) of Cu and Te between a Mo electrode and a CdTe absorber. US Published Patent Application No. 20090235983 A1 teaches forming a thin highly doped p-semiconductor before depositing the CdTe absorber layer.
None of the prior art solutions solves the problem of creating a successful ohmic contact satisfactorily in either substrate or superstrate configuration solar cells. Accordingly there is a need for an improved solar cell having increased efficiency due to the improved ohmic contact between the absorber layer and the metal electrode layer.